I prefer painted backgrounds over 3D or drawn, but uh... that's like... NOT REALISTIC
So I just apply a painterly filter over my 3D renders and paint over them a little bit where it matters lmao. Also, +1 with what JeanGuillet said about tone curves. If you're using 3D renders, you better get reeeeaaaal friendly with tone curves and gradient maps.
I'm still learning how to use the tone curve effectively, hopefully I'll master it one day 
These are some of my recent 3D rendered backgrounds. Tone curves, gradient maps, and also a bit of paint. Drawing white lines on harsh edges and lowering the opacity to match is the best thing you can do to make the 3D less harsh. I also recommend against using realistic textures on models. Just smudge out detailed textures in the painting phase if it can't be avoided.
Edit: If anyone wants a good tutorial on learning tone curves specifically for backgrounds, this tutorial is AMAZING. This artist specializes in more anime/Studio Ghibli style, but even if that's not your thing, this is still a must read. https://www.mclelun.com/2019/04/colour-grading-using-curves-and-levels.html